1. Field of the Invention
The present invention relates to active and semi-active hydraulic suspension systems for isolating a component, such as an operator cab or a seat, from vibrations in other sections of a vehicle while traveling over rough terrain; and more particularly to such hydraulic suspension systems which incorporate automatic load leveling.
2. Description of the Related Art
Vibration has an adverse affect on the productivity of work vehicles in which an operator cab is supported on a chassis. Such vehicles include agricultural tractors, construction equipment, and over the road trucks. The vibrations experienced by such vehicles reduce their reliability, increase mechanical fatigue of components, and most importantly produce human fatigue due to motion of the operator's body. Therefore, it is desirable to minimize vibration of the vehicle cab or the seat in which the operator sits and of other components of the vehicle.
Traditional approaches to vibration mitigation employed either a passive or an active suspension system to isolate the vehicle cab or seat along one or more axes to reduce bounce, pitch, and roll of the vehicle. Passive systems typically placed a series of struts between the vehicle chassis and the components to be isolated. Each strut included a parallel arrangement of a spring and a shock absorber to dampen movement. This resulted in good vibration isolation at higher frequencies produced by bumps, potholes and the like. However, performance a lower frequencies, such as encountered by a farm tractor while plowing a field, was relatively poor. The lower frequency vibrations can be in the same range as the natural frequency of the passive suspension system, thereby actually amplifying the vibration. Therefore, such previous vehicle suspension systems often performed poorly in the range of vibration frequencies to which the human body is most sensitive, i.e. one to ten Hertz.
Active and semi-active suspension systems place a cylinder and piston arrangement between the chassis and the cab or seat of the vehicle to isolate that latter component. The piston divides the cylinder into two internal chambers and an electronic circuit operates valves which control the flow of hydraulic fluid between the chambers.
U.S. Pat. No. 4,887,699 discloses an semi-active vibration damper in which the valve is adjusted to control the flow of fluid from one cylinder chamber into the other chamber. The valve is operated in response to one or more motion sensors, so that the fluid flow is proportionally controlled in response to the motion.
U.S. Pat. No. 3,701,499 describes a type of active isolation system in which a servo valve selectively controls the flow of pressurized hydraulic fluid from a source to one of the cylinder chambers and controls exhaustion of oil from the other chamber back to a tank supplying the source. A displacement sensor and an accelerometer are connected to the mass which is being isolated from vibration and provide input signals to a control circuit. In response, the control circuit operates the servo valve to determine into which cylinder chamber fluid should be supplied, from which cylinder chamber fluid should be drained and the rate of those respective flows. This application of pressurized fluid to the cylinder produces movement of the piston which counters the vibration.
For optimum vibration damping, the piston should be centered between the cylinder ends under static conditions. However, the piston may drift toward one end of the cylinder due to changes in the load on the vehicle. A similar drift occurs during prolonged vibrating conditions, such as when an agricultural tractor is plowing a field. Other effects, such as leakage of hydraulic fluid and friction between the piston and the cylinder, also affect the position of the piston under static conditions. To compensate for that piston drift, prior suspension systems included a sensor that indicated the distance between the vehicle components to which the cylinder/piston rod combination was connected and thus provide an indication of piston drift within the cylinder. In response to that signal, main control valve was opened to apply more fluid into one of the two cylinder chambers and exhaust fluid from the other chamber under static conditions to re-center the piston.
However this type of load leveling increased the power requirements of the active suspension system because the dynamic response has to overcome the weight of the supported mass with each activation. This requires that the pump of the vehicle's hydraulic system operate above the normal standby pressure that occurred otherwise when other hydraulic devices were not being operated, such as when the vehicle was being driven along the ground.